Passage 14
Nearly a century ago, biologists found that if they
separated an invertebrate animal embryo into two parts
at an early stage of its life, it would survive and develop
as two normal embryos. This led them to believe that the
(5) cells in the early embryo are undetermined in the sense
that each cell has the potential to develop in a variety of
different ways. Later biologists found that the situation
was not so simple. It matters in which plane the embryo
is cut. If it is cut in a plane different from the one used
(10) by the early investigators, it will not form two whole
embryos.
A debate arose over what exactly was happening.
Which embryo cells are determined, just when do they-
become irreversibly committed to their fates, and what
(15) are the “morphogenetic determinants” that tell a cell
what to become? But the debate could not be resolved
because no one was able to ask the crucial questions
in a form in which they could be pursued productively.
Recent discoveries in molecular biology, however, have
(20) opened up prospects for a resolution of the debate.
Now investigators think they know at least some of the
molecules that act as morphogenetic determinants in
early development. They have been able o show that,
in a sense, cell determination begins even before an egg
(25) is fertilized.
Studying sea urchins, biologist Paul Gross found
that an unfertilized egg contains substances that func-
tion as morphogenetic determinants. They are located
in the cytoplasm of the egg cell; i.e., in that part of the
(30) cell’s protoplasm that lies outside of the nucleus. In the
unfertilized egg, the substances are inactive and are not
distributed homogeneously. When the egg is fertilized,
the substances become active and, presumably, govern
the behavior of the genes they interact with. Since the
(35) substances are unevenly distributed in the egg, when the
fertilized egg divides, the resulting cells are different
from the start and so can be qualitatively different in
their own gene activity.
The substances that Gross studied are maternal
(40) messenger RNA’s --products of certain of the maternal
genes. He and other biologists studying a wide variety
of organisms have found that these particular RNA’s
direct, in large part, the synthesis of histones, a class
of proteins that bind to DNA. Once synthesized, the
(45) histones move into the cell nucleus, where section of
DNA wrap around them to form a structure that resem-
bles beads, or knots, on a string. The beads are DNA
segments wrapped around the histones; the string is the
intervening DNA. And it is the structure of these bea
9. Which of the following circumstances is most
comparable to the impasse biologists encountered in
trying to resolve the debate about cell determination
(lines 12-18)?
(A) The problems faced by a literary scholar who wishes
to use original source materials that are written in
an unfamiliar foreign language
(B) The situation of a mathematician who in preparing a
proof of a theorem for publication detects a
reasoning error in the proof
(C) The difficulties of a space engineer who has to
design equipment to function in an environment in
which it cannot first be tested
(D) The predicament of a linguist trying to develop a
theory of language acquisition when knowledge of
the structure of language itself is rudimentary at best
(E) The dilemma confronting a foundation when the
funds available to it are sufficient to support one of
two equally deserving scientific projects but not both
不明为什么选D. 谁可以把那句抽象的话明确点啊. 谁可以解释一下二者的是如何类比的呢.
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